The present invention relates to electronic voltage regulators and, more particularly, to an integratable electronic series regulator which has a minimum input-output voltage drop and has PNP bipolar power output pass transistor.
A voltage regulator is inserted between a generator and a load with the object of obtaining a continuous voltage across the load which has a preset value and which is substantially independent--within preset limits--from the current passing through the load.
In this type of regulator, it may be necessary to protect the pass transistor, through which flows the current which passes through the load, so as to enable the transistor to operate within the safe operating area (S.O.A.).
A number of devices are known for the protection of NPN power transistors which ensure operation in the S.O.A. under different overload conditions.
For example, reference is made to the solutions set out in paragraph IV--"Short circuit and overvoltage protection" of the article "Power Integrated Circuits: Problems, Tradeoffs and Solutions" published in the "IEEE Journal of Solid State Circuits," Vol. SC13, No. 3, June 1978 and, more particularly, to the protective device shown in FIG. 8 of the article and illustrated in FIG. 1 of the drawings accompanying the present application, in which the voltage and current of the power transistor are monitored continuously and simultaneously. In order to obtain a voltage drop which is proportional to the current flowing through the load, this solution uses a resistance element (a gold wire RW) in series with the power transistor.
This solution, although normally also applicable to PNP transistors, has the disadvantage that the series resistance produces a voltage drop that cannot be tolerated in applications whose purpose is to minimize the input-output voltage drop of the regulator. Therefore, the prior art is inadequate in cases where it is necessary to ensure protection within the S.O.A. for the PNP power transistor and at the same time have a minimum series voltage drop thereacross.